1. Field of the Invention
The present invention relates to a method of absorbing thermal energy at a low temperature, and more particularly to a method of efficiently absorbing thermal energy at a temperature below the critical temperature of a working medium in a refrigerating machine such as, for example, a Stirling-cycle refrigerating machine having a compressor, a radiator, a heat exchanger and an expander.
2. Discussion of the Background
One known method of absorbing thermal energy at a low temperature is disclosed in Japanese Patent Application published on May 4, 1976 as Publication No. 51-13900. The prior art method continuously maintains the pressure of a working medium higher than a pressure substantially equal to the critical pressure in a refrigerating machine having a compressor, a radiator, a heat exchanger, and an expander, and lowers the temperature of the expander below the critical temperature of the working medium for thereby absorbing the thermal energy. With the prior art method, however, the thermal energy cannot efficiently be absorbed since no condition is varied if the working medium is expanded for heat absorption at a temperature lower than the critical temperature. Such a condition will be described with reference to the T-S diagram shown in FIG. 1 of the accompanying drawings, in which helium is employed as the working medium. An amount QE of absorbed heat which is generated by expansion of the working medium in the expander and various amounts of mechanical work W transferred from an external source to the working medium for absorbing the heat are indicated by areas enclosed by a2, a2', a3', a3 and a1, a2, a3, a4, respectively. The externally applied amount of work W is close to the critical pressure in the T-S diagram, and is highly reduced in a low-temperature region below approximately the critical temperature, thus reducing the absorbed amount of heat QE.
Accordingly, the diagram indicates that COP (coefficient of performance)=QE/W representative of the efficiency of thermal energy absorption is greatly reduced. Where the working medium is a helium gas, the lowest pressure is 3 atm, the pressure ratio is 3, the temperature of the compressor is 10.degree. K., and the temperature of the expander is 4.2.degree. K., the COP is about 12%.